fix(streaming): clear data stream on subject completion

public/app/features/panel/metrics_panel_ctrl.ts

@@ -252,6 +252,7 @@ class MetricsPanelCtrl extends PanelCtrl {
       },
       },
       complete: () => {
       complete: () => {
         console.log('panel: observer got complete');
         console.log('panel: observer got complete');
+        this.dataStream = null;
       }
       }
     });
     });
   }
   }